TUESDAY, June 25, 2019 (HealthDay News) -- The U.S. Food and Drug Administration on Tuesday took two kratom marketers to task over false claims that their products can treat or cure opioid addiction.

The latest salvo is part of the agency's continuing efforts to alert consumers about the potential dangers of the herbal drug.

The warning letters were sent to Cali Botanicals of Folsom, Calif., and Kratom NC of Wilmington, N.C., for illegally selling unapproved, misbranded kratom-containing drug products with unproven claims about their ability to treat or cure opioid addiction and withdrawal symptoms.

The companies also make unproven claims that the products treat pain and other medical conditions such as depression, anxiety and cancer.

The companies were given 15 working days to outline what actions they'll take to address the FDA's concerns. Failure to act may result in product seizures or injunctions.

"We have issued numerous warnings about the serious risks associated with the use of kratom, including warnings about the contamination of kratom products with high rates of salmonella that put people using kratom products at risk, and resulted in numerous illnesses and recalls," Acting FDA Commissioner Dr. Ned Sharpless said in an agency news release.

"As part of our efforts to assess kratom for contamination in the products tested, we also found high levels of heavy metals in kratom products. Despite our warnings, companies continue to sell this dangerous product and make deceptive medical claims that are not backed by science or any reliable scientific evidence," Sharpless added.

"As we work to combat the opioid crisis, we cannot allow unscrupulous vendors to take advantage of consumers by selling products with unsubstantiated claims that they can treat opioid addiction or alleviate other medical conditions," he said.

Kratom (Mitragyna speciosa), a plant that grows naturally in Thailand, Malaysia, Indonesia and Papua New Guinea, is not legally marketed in the United States as a drug or dietary supplement. There are no FDA-approved uses for kratom, and the agency said it has received reports about dangers posed by kratom.

Data suggest that certain substances in kratom have opioid properties that put users at risk of addiction, abuse and dependence, according to the FDA.

A U.S. Centers for Disease Control and Prevention study published in April found that over an 18-month period (July 2016 through December 2017), kratom was a contributing factor in 91 drug overdose deaths.

In seven of those cases, kratom was the only drug detected in postmortem testing, according to the study that examined data from 27 states.

Along with those 91 deaths in which kratom was at least a contributing factor, 61 other people who suffered fatal drug overdoses were found to have kratom in their bloodstreams, although other drugs may have caused the deaths, the CDC researchers said at the time.

The FDA warns consumers not to use any products labeled as containing the botanical substance kratom or its psychoactive compounds, mitragynine and 7-hydroxymitragynine.

The U.S. Drug Enforcement Agency has also listed kratom as a "drug of concern."